In general, thermoplastic polyesters, e.g. polyethylene teterephthalate, have superior dynamic and chemical properties and are in wide use as shaped articles such as films and fibers.
However, polyesters have involved a problem of deterioration in productivity due to poor sliding property thereof at the time of processing into shaped articles. For remedying this problem, there has been adopted heretofore a method wherein inactive particles are dispersed in polyesters to give uneven surfaces of the resulting shaped articles. For example, in Japanese Patent Laid Open No.86471/1977 (JP52-864791A) there is proposed a method using inorganic particles of a specified specific surface area, and in Japanese Patent Laid Open No.171623/1984 (JP59-171623A) there is proposed a method using spherical colloidal silica of 0.1 to 1.mu. in diameter. These methods are effective in solving the problem of poor sliding property, but shaped articles obtained by using the compositions described therein are not at a satisfactory level in point of abrasion resistance and scratch resistance.
In the case where the abrasion resistance of a shaped article, e.g. film for magnetic tape, is low, abrasion dust of the film is apt to be produced during the manufacture of the magnetic tape, and drop-out of application occurs in a magnetic layer applying step, this causing drop-out of magnetic recording, etc. When a magnetic tape is used, in many cases, the tape is allowed to travel in contact with a recording/reproducing apparatus or the like, so abrasion dust produced at the time of such contact is deposited on a magnetic material, thus causing drop-out of magnetic recording at the time of recording or playback.
In the case where the scratch resistance of a shaped article, e.g. film for magnetic tape, is low, a foreign matter if present during the manufacture of the magnetic tape would easily scratch the film surface, resulting in drop-out of magnetic recording or easy scratching of the film surface during high-travelling of the magnetic tape.
In the case of film for magnetic tape, it is necessary to have sliding property, abrasion resistance and scratch resistance not only during the manufacture of the magnetic tape but also in use as the magnetic tape. For improving the abrasion resistance, there has been proposed a method of incorporating fine particles in polyester, for example, dispersing in polyester a fine spherical colloidal silica having an average particle diameter of 0.01 to 0.1 .mu.m. In this case, the particles may drop out due to a strong force exerted thereon from a roller with which the film formed by using such polyester is in contact, and the particles thus dropped out scratch the film surface.
The present inventors have found out that the kind, size and shape of particles contained in a polyester film are predominant factors which influence the abrasion resistance and scratch resistance of the film. And as a result of earnest studies, we accomplished the present invention.
Silicas having special particle shapes are disclosed in U.S. Pat. Nos. 2,680,721 and 2,900,348 and Japanese Patent Laid Open No.317115/1989 (JP1-317115A) (corresponding to European Patent No.0335195), but these publications are concerned with the preparation of mere silica sol. Prior to the present invention it had not been found out that a polyester-silica combination would afford a film, especially a film for magnetic tape, superior in sliding property, abrasion resistance and scratch resistance.
The present invention has been accomplished for eliminating the above-mentioned drawbacks of the prior art and it is the object of the invention to provide thermoplastic polyester compositions capable of affording films and fibers superior in all of sliding property, abrasion resistance and scratch resistance.